The purpose of this research is to understand some of the principles governing the structure and function of chromosomes, with emphasis on the chromosomes of Drosophila. Specific objectives concerning chromosome structues are, (1) How are chromatin fibers organized to give the specific chromosome morphology; (2) What structural rearrangements occur during induction of RNA synthesis; (3) What genes control the polytenization process, and how? (4) Does differential polyteny extend to interband sequences? (5) How is differential polyteny controlled? Specific objectives concerning chromosome function are: (6) What structural feature of polytene chromosomes controls dosage compensation? (7) Does position-effect variegation occur by transcription repression and/or by gene loss? (8) How does Drosophila control activity at sites of transcription initiation and termination? (9) What are the function(s) of the regions in polytene chromosomes that contain sequences complementary to actin mRNA? Methods and approaches include morphological and structural (electron microscopy, optical diffraction, image reconstruction), biochemical and cytogenetical (renaturation kinetics, in situ hybridization), and genetical (production and characterization of mutants) techniques.